The Encyclopedia of Religion (edited by Mircae Eliade) has much to say about
Esus. What follows are all the entries mentioning Esus from the 1987 version of
the EoR (The entries are separated by paragraph; listed in blue at the beginning
of each paragraph is the encyclopedia, the name of the entry, the volume, and
the pages that particular paragraph appears on):

Encyclopedia of Religion, CELTIC RELIGION, Vol.3, p.153
- p.154Gaulish Mars. A famous passage in Lucan's Civil War refers
to the bloody sacrifices offered the three Celtic gods Teutates, Esus, and
Taranis. A later commentator on Lucan clearly illustrates the difficulty of
identifying individual Gaulish and Roman gods, for one of his two main sources
equated Teutates with Mercury, the other with Mars. But if, as seems likely, teutates
is primarily a title ("god of the tribe") rather than a name, then
such confusion is explainable: the god of sovereignty and the arts, "Mercurius,"
will also function as a warrior, while the god of war, "Mars," will
often function as the protector of the tribe. Consequently, their functions will
sometimes overlap, and it may be a matter of chance or circumstance which is
given preeminence in a given time or place. A further complication is that many
of the Gallo-Roman dedications to Mars present him not only as a god of war but
also as god of healing and guardian of the fields, but this may reflect an
extension of his role in the Roman period and does not necessarily discredit
Caesar's description of him as god of war. So far as the insular tradition is
concerned, a god of war does not come into clear focus, perhaps because fighting
is a more or less universal, rather than a differentiating, feature in the
heroic context. Thus one cannot easily define the role of Mars, and one cannot
so easily assign him a pan-Celtic identity as one can Lugh.

ESUS

Encyclopedia of Religion, ESUS, Vol.5, p.167 ESUS, a Celtic deity of uncertain attributes, was variously identified by the Romans with Mars and with Mercury. As a theonym
Esus is unknown in Gallo-Roman writing except for a single inscription from the first century CE on the Altar of the Nautae of Paris, currently preserved at the Musée de Cluny
(Corpus inscriptionum Latinarum, Berlin, 1863, vol. 13, no. 3026). But the name is indirectly attested in several godlike an-throponyms:
Esumagius ("he who is as powerful as Esus"), Esugenus ("son of Esus"), and
Esunertus ("he who has the strength of Esus"). There are also several ethnic names with related etymologies, such as
Esuvii, Esubii, Esuii, and Esubiani (Vesubiani).

Encyclopedia of Religion, ESUS, Vol.5, p.167
The name Esus appears in the Pharsalia of Lucan (1.444-446), at the center of the "Jupiterean" triad: "Et quibus inmitis placatur sanguine diro, / Teutates horrensque feris altaribus Esus / Et Taranis Scythicae non mitior ara Dianae . . . ("And those who appease ferocious Teutates with horrible bloodletting, hideous Esus in his savage sanctuaries, and Taranis, whose altars are no less cruel than those of Scythian Diana . . ."). Esus is also mentioned much later in the Bernese Scholia, which propose two successive interpretations of him. One reads: "Hesus Mars sic placatur: homo in arbore suspenditur usque donec per cruorem membra digesserit" ("Hesus-Mars is appeased in this mannner: they hang a man from a tree until his members fall off from the loss of blood"). The other reads: "Hesum Mercurium credunt, siquidem a mercatoribus colitur, et praesidem bellorum et caelestium deorum maximum Taranin lovem adsuetum olim humanis placari capitibus" ("They think that Esus is Mercury, although he is honored by the merchants, and as war leader and great god of the heavens they have Taranis-Jupiter, who was once appeased by human heads but who is now content with herds").

Encyclopedia of Religion, ESUS, Vol.5, p.167
The sculpture from the Altar of the Nautae shows him felling or pruning a tree, and he has often been called "woodcutter god," which is a rather dubious interpretation. It is much more likely that the pruning or lopping off of the tree is associated with the birds of Tarvos Trigaranus ("bull of the three cranes") depicted on the same monument. Esus also shows up on a monument in Treves, although here too one cannot be sure of a sound interpretation.

Encyclopedia of Religion, ESUS, Vol.5, p.167
The etymology of Esus has for a long time been subject to doubt, and parallels from Sanskrit or Latin (e.g.,
herus, "master") have been sought. But it would appear that the etymology is simply a case of the evolution of the stem
*veso-s through the disappearance of the initial v-, as frequently happened in all the Celtic languages. In any case, the etymology is proved by the coexistence of the ethnic names
Esubiani and Vesubiani. *Veso-s denotes "better, excellent," and Esus is thus the Celtic semantic equivalent of the Latin
Optimus, an epithet for Jupiter that also resembles the Irish Daghdha.

Encyclopedia of Religion, TARANIS, Vol.14, p.339
Taranis is cited in only one literary text, the Pharsalia of Lucan (1.444-446).
[See Esus and Teutates.] Lucan's Bernese Scholium identifies Taranis with Dis Pater and attributes to him sacrifice by fire: "Taranis Ditis Pater hoc modo aput eos placatur: in alveo ligneo aliquod homines cremantur" ("They appease Taranis Dis Pater in the following manner: they burn a certain number of men in a wooden cage").

Encyclopedia of Religion, TEUTATES, Vol.14, p.409TEUTATES, a Celtic deity of uncertain attributes, was variously identified by the Romans with Mars and with Mercury. The theonym
Teutates is attested in Lucan's Pharsalia (1.444-446), together with
Taranis and Esus: "Et quibus inmitis placatur sanguine diro / Teutates horrensque feris altaribus Esus / Et Taranis Scythicae non mitior ara Dianae" ("And those who appease ferocious Teutates with horrible bloodletting, hideous Esus in his savage sanctuaries, and Taranis, whose altars are no less cruel that those of Scythian Diana").

Encyclopedia of Religion, TEUTATES, Vol.14, p.409 - p.410
Joseph Vendryes once explained the name by a haplology of *touto-tatis ("father of the tribe"). But this hypothesis is unnecessary:
-atis is a suffix of anthroponymic, ethnic, and geographic derivation widely attested in all the Celtic languages. Etymologically it means "man of the tribe" or "man of the city," or again, because of a morphological analogy duplicating a taboo, "the man from the north," in which case the name is clear allusion to the Nordic and polar origins of Celtic tradition. It is with this sort of semantic reasoning that this god of the "community" can be better understood. But since there are no documents, it is difficult to say more about him. (Unlike Taranis and Esus, Teutates is not attested in Gallo-Roman iconography.) The Teutates-Esus-Taranis triad in Lucan still awaits a clear explanation beyond its evident Jupiter-like character.